Double-walled drill pipe as used in reverse-circulation drilling has been known for many years, as evidenced by U.S. Pat. No. 1,909,075 (Ricker et al, May 16, 1933) and Canadian Patent Nos. 630,100 (Grable, Oct. 31, 1961); 753,528 (Henderson, Feb. 28, 1967); 841,463 (Ellenburg, May 12, 1970); and 854,056 (Sandquist et al, Oct. 20, 1970). Each of these patents describes a drilling system using a double-walled pipe which uses an outer pipe and a concentric inner pipe which creates an annular flow passage between the pipes. A light fluid or gas such as air may be forced down from the surface through the annular flow passage to the cutting bit. This fluid then cools the bit and collects fragments and chips which result from the bit cutting into the ground. These cuttings are then carried by the fluid through a central passage in the inner pipe to the surface. The function of the outer pipe is to transmit torque and a vertical force down the drill hole to the bit, and to form the outer barrier of the annular passage. The function of the inner pipe is to form the inner wall of the annular passage and to form the outer boundary of the central passage. There is usually a connection between inner pipe sections of a made-up drill string to provide continuity of the annular and central passages. Such a connection is shown in Canadian Pat. No. 854,056.
In each of the above-identified patents the inner pipe is concentrically located in the outer pipe at a plurality of areas along its length and is fixed, as by welded ribs to the outer pipe to provide fixed connections at one or both ends which precludes relative rotation at the connection between the inner and outer pipes.
In rotary drilling operations, double wall pipes are subject to relatively high torque conditions resulting in variable circumferential angular deflection (twist) of the pipe and variable relatively high torsional stresses transmitted between the outer pipe and the inner pipe through the welded rib connections therebetween. Consequently, there have been failures in the double wall pipe at or adjacent to the area of the welded connections resulting from torque induced metal fatigue and/or excessively high torque conditions. If the inner pipe is of smaller guage with a thinner wll thickness or of a material of lesser strength that the outer pipe, the failure is most likely to occur in the wall of the inner pipe adjacent to the welded connection between theinner pipe and the ribs.